Aftermarket head unit not working

[Velocity502]

So I've tried 3 different head units in my 88 silverado and none seem to want to work. As far as I know I've wired the adapter correctly and I've even checked the fuses. The most that has happened is that the display on one of the units barely lit up. No matter what I try, I'm unable to find a solution

 

[Zerio29]

you may have to run a separate ignition controlled switch power from the fuse box. Best way to check is to put a voltmeter on the switch power from the truck and verify that you're actually getting 12v there.

 

[theunderlord]

if the display was dim, but actually lit up, you could have a grounding problem.