Put flatbed.dFF and flatbed.tXD in a folder inside of your modloader folder or remplace the files in your gta3.img...
Remplace the lines in handling.cfg, vehicles.ide and carcols.dat respectively:

Handling:
FLATBED      5680.0   11360.0 	2.5	   0.0 0.15 -0.2 100 0.65 0.7  0.47 	5 100 	25.0 80.0 R D 	8.0   0.30 0 29.0 	1.0  0.06  0.0   0.45 -0.25 0.55 0.3 		0.48 1.0  40000 	220049 		300001 		1  1 	13 	

Vehicles line:
455, 	flatbed, 	flatbed, 	mtruck, 	FLATBED, 	FLATBED, 	van,	worker,		2, 	0,	0,		-1, 1.18, 1.18,		0

Colors line:
flatbed, 86,86, 92,92, 36,36, 43,31, 54,54, 114,114, 108,108

-IonicCleric8076-