A bad ground should make the headlights dim it should have no effect caused by turning on ign. Headlights are not fused they normally run a fusible link or an auto reset breaker. The floor dimmer on a chevy truck has 3 wires to it one is the power coming from the headlight switch the other 2 go to the seal beams one is high beam power the other is low beam power. the dimmer switch is not ground effected.
In 70 series chevy trucks I have run into many that do to resistance melted the plastic connector on the back of the switch causing all kind of weird things to happen. this is the first place that I would look. If your headlights are turning off and on with the ignition you either have a short or someone has miss wired your system.
The main reason to try to find the problem is if you are pulling power from another fused circuit you could be driving at night and with the addditional circuit load blow a fuse. Then no headlights. That could be fun on a dark country road in the rain. Your tombstone could read "I should have fixed my wiring".
Seriously you need to fix it. If your library has a copy of the GM service manual for your truck or if they subscribe to a system of computer manuals the should have information you need. with a flow chart for tracking down the problem. Also you mentioned you had thighten the bezel check if it moved things enough to cause your problem then adjust or repair fault then retighten.
Good Luck
Been there, Done that, Hope to live long enough to do it again.
Big Bill
I.I.# 4698
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