this looks very promising. there are two grounds for the lighting system on the front side of the radiator support. they are just phillips heads screws that go through a loop on the end of the wire. these are exposed to the elements as you drive. i would take those screws off and clean up the metal under the loops and the loops them selves with some sand paper or emery cloth. if this doesnt fix the problem, it can at least help with the brightness of the bulbs using these grounds. another thing you can do is find some star type lock washers like these and put them between the loop and the radiator support. the edges will dig in and give a good solid ground. then you can paint over it to prevent the corrosion from forming.
I found that if you push the bulb straight back ( use a rag or piece of cardboard in case the bulb breaks ) then turn it as you release the pressure, it will come out easier! The " concave " you mentioned comes from years of the spring pressure and heating and cooling of the soft lead the contacts are made of. The bulbs are made originally with either flat or convex contacts. The dimmer is in no way connected to your turn signal indicator! It dims only the instrument panel lights. Two totally different circuits. All the red indicator lights are also on separate circuits.
Finally found it, the driver’s front side marker lamp seems to be the issue. I removed the bulb and plugged in back in several times. Have not seen the issue again. The bulb appeared to be a little corroded. I’ll get a new bulb and likely a new socket down the road. Certainly appreciate all the help, just took me several weeks to try this and that until I could narrow it down. kb
