Most ppl makes a muffler slot on the chassis or on the rear bumper
Paste in the main.cfg where are the other slots
slot 0.440 -0.140 -0.044 0.000 0.000 0.000 2 ; L muffler
attach 0x00040047 1
slot -0.440 -0.140 -0.044 0.000 0.000 0.000 3 ; R muffler
attach 0x00040047 1
Harrison's way is the proper, that's the quick and dirty