Stock trim they usually ground to the engine block through the housing.
To upgrade add a heavy gauge wire to the frame or negative battery terminal
Stereo guys running big amps do the “Big 3”
- heavy gauge charge wire
- heavy gauge Alt ground to battery
- heavy gauge body ground
Or the big 5 which adds
- heavy gauge engine ground
- heavy gauge ground from battery to frame
Here’s mine partially done, one of the blue 4ga cables to the batt lug of the alternator and the alternator ground is 4ga to the frame. 0ga cables from the frame and fuse block to the battery.
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