Try this:
Code:
stock fm_left_velocity(index, multiplier, Float:velocity[3]) {
new Float:vec[3]
pev(index, pev_v_angle, vec)
engfunc(EngFunc_MakeVectors, vec)
global_get(glb_v_right, vec)
velocity[0] = -vec[0] * multiplier
velocity[1] = -vec[1] * multiplier
velocity[2] = -vec[2] * multiplier
return 1
}