Having two electrical issues with my F-250.

1) The temperature gauge is always to the far right, as far as it can go. When I started using the truck a few months ago, the gauge didn't seem to work at all - moving slightly when the truck is started, but never really levelling out in the normal operating temperature range. The truck is standard, so one day when I pulled the lever to disengage the parking brake, the "bump" from the parking brake pedal must've unstuck or altered some connection because it randomly started working. It was short-lived, as after about a day or two it stuck to the far right completely, and has stayed there ever since. Any ideas on what to look for in terms of troubleshooting?

2) I had enough of the cassette player. Swapped it out for a Sony Xplode stereo. All wiring was done really well and I know there aren't any stray wires touching that shouldn't be. When I finished the job, the stereo works fine, but the volt meter started to fluctuate back and forth, and the battery light flickers on and off. You can notice jumps in the volt meter when you use other accessories (heat, lights, etc.). At first, I tightened the negative lug on the battery just in case I had it left too loose. It stopped the issue. But after leaving the truck overnight and starting it in the morning, the issue has come back. I can't imagine the new stereo puts that much draw on the battery. Has anyone else run into this issue or have any idea what the cause may be?

Thanks in advance!