EDIT 14.2.2018
Welcome fellow Burnin' Rubber 5 HD fanatics! It seems like we're in luck, the location of the packages seem to match! I'll be updating this post if I find any changed locations.
Original 2013 post follows:
Challenge 1: Speedway - "Pitstop" area. At the first bend, do a 180° turn and keep right
Challenge 2: Line 6 - At the first opening, on the far side of the first train.
Challenge 3: Slipstream - Starting at the last bend, there is a similar pit stop area as on the first Burnin' Rubber 5 Challenge. Here aswell, do a sharp 180° turn and keep right, past the truck that blocks the entrance.
Challenge 4: Destruction derby - Same bend as the challenge before, at the beginning, next to the truck that blocks the entrance.
Challenge 5: Shipwrecked - Two Tube Tunnels a few bends before the finish line, the sixth pack is in the right hand one.
Challenge 6: Pelham one Two Three - In the tunnel, on the wall-side of the first train that's in the road. No need to try and pry your way between the concrete blocks.
Challenge 7: Death Race - Coming out of the tunnel, up on the bridge, on the wall-side of the first train on the left.
Challenge 8: Oilrig - Coming out of the tunnel, take the exit to the right, behind the bus and the crates.
Challenge 9: Heat - At the first sharp turn, keep turning another 90° and find the pack at the protruding container.
Challenge 10: Deception - After going one level up on the stranded ship(?), after the first bend and during the straight part, there are two "rooms" on the left side. The first one is filled with sand and not accessible. The second is accessible and this is where you will find the package.
Challenge 11: Avalanche - Just after the strat, you see two police cars on the left. Between these two, there is a package and a very familiar looking tunnel that has collapsed since we last saw it.
Challenge 12: Monster Madness - Behind the container to the left of the tunnel entrance after the slope after start.
Challenge 13: Roughneck Rampage - After the sharp S bend before the big opening about 3/4 of the course, turn another left even as the big opening wants you to go straight forward.
Challenge 14: Rush Hour - Behind the single gas tank on the right, where there is a broad, car-accessible sidewalk and a bus stop.
Challenge 15: Blizzard - Behind the fuel truck and car just before the short snowy part just before the finish line
Challenge 16: Camper Crash - On the left hand side of the snowy part just before the finish line.
Challenge 17: San Burrito Airport - On the right side right outside the airport main entrance, there is quite a surprising parking hall one floor down. Before the first outer building/pillar base, there is a ramp to go down. You will have to past the pillar base, though. When you get down, the pack is directly to the right.
Challenge 18: Johnny Cabs -
Challenge 19: School's out - Not very hidden this one, but there you go. Under the bridge you go under twice, you'll pick the pack the second time around, but you very well see it the first time.
Challenge 20: Super Offroad - Behind the rock on the right just before the beach that leads to the finish line.
Challenge 21: One Four Eight - Turn a sharp left directly when you enter the parking garage. No, not around the first bend. Right away.
Challenge 22: Assault - Where the highway has fallen, and the road turns into a short dirt slope onto the tarmac below, turn around, go past the dirt slope to the left, and turn right when it allows you to.
Challenge 23: Hard Boiled - The half-fallen bridge has a school bus on it. The package is on the right side of that bus.
Challenge 24: Devils Run - Directly at the bottom of the waterfall jump, you'll find the 24th hidden package.
Challenge 25: APC - In the grass on the left side, before the bridge.
Challenge 26: Shipyard - In the short tunnel on the right side just after start.
Challenge 27: Street Life - After the fallen bridge, there is a big sign that points to the right. This sign somewhat hides a little passage. The 27th pack is in there.
Challenge 28: Total Recall - On the right side of the road at the last bend before the finish line. Not too hidden this one.
Challenge 29: Barrage Dam - After the three gas tanks to the right, and opposite the four large pipes on the left wall, there is a somewhat narrow opening. Go in there, keep left and you'll find the next package.
Challenge 30: Inferno - The big opening before the finish line has a ramp on each side. The package is found is on the left one, behind one of the pillars.
Challenge 31: Uncle Sam - There's a huge ship put "on dock" here. Look on the left side of it, on the far side of the last base that the wire from the ship goes to.
Challenge 32: Railway Station - Where there are lots of trains, directly keep left past the one train. There is the a ramp-type of carriage. The top of the package can be seen about halfway on this carriage.
Challenge 33: Flood Control - Under the first ball shaped, large containers on the left just after start is where you'll find the 33rd package.
Challenge 34: Area One - After the jump throug the open train, keep left and find the package to the right just before the string of cask pick-ups.
Challenge 35: Panzer - The pack is on the same side as, and kissing the wall just after the four large tubes on the left after start.
Challenge 36: Operation Sandstorm - Somewhat tricky, this one. To the right after the long, straight tunnel, you'll find the 36th package.
Challenge 37: - Behind the left turret on the top floor at the rear of the.. thing..
