I have a 2001 GMC Sierra 2500HD, and a new problem recently occurred with my turn signals, where they continuously remain on but only when the DRL is on. Also, I have lost my tail lights and my reverse lights, and the third brake light is half lit. When it is during the day, I get reverse lights, but only the reverse pod lights I have spliced into the reverse bulb wiring and not the actual bulb in the tail light, and the turn signals on my dash light up dim when I put the truck in reverse. At night, when the DRL is on I get no reverse lights. I have been reading around to try and figure out what could be my issue but it seems that my problem is too specific. I replaced the flasher relay along with checking my fuses. Any suggestions?

Clean the grounds and the body in particular.

Yeah, what carfixer007 said ^^^^^^^ and I would start by tracing grounds related to all of the lights on the back of your vehicle.

Thanks for the response. I took apart the dash and unplugged the body control module and the plug on the far right had some corrosion on it so I cleaned it up with some electrical cleaner and it seemed to fix it last night until I got back in the truck this morning and I had the same issue.

Those pins are very sensitive to any corrosion. A long term fix is to take the pins out and clean them to a shine or replace. There are cheap tools you can buy to take connectors apart.
You can do it with thin pick tools if you have any. They usually have two legs that need to be pushed in to release it.

From above.
Those pins are very sensitive to any corrosion. A long term fix is to take the pins out and clean them to a shine or replace. There are cheap tools you can buy to take connectors apart.
You can do it with thin pick tools if you have any. They usually have two legs that need to be pushed in to release the pin. Males and females are similar but the females are hard to get clean. These you may want to replace.
Low voltage circuits that feed these modules need to be very clean. If you can see corrosion at all it needs to be addressed. I've seen problems with pins that were just dull and not corroded. Very sensitive stuff.

So I did the same cleaned it up. And no luck so i decided to run to the back of the truck and same deal with the plug block in the back for the tail lights and just pushing it up a hair fixed my problem. Must’ve been jiggling around when driving. Thanks for all the help guys

Thanks for letting us know what fixed it!