Do you mean the 3rd brake light is completely original? Because many have found the original to be pretty poor at keeping out water.

 

I had the same issue, about the same severity as yours. I removed my third brake light and added a new weather stripping to it and re-installed it (found this fix on the dodge ram forums) . I then used a paint stripping wheel to strip the cab corners down to bare metal and it also removed the corrosion under the paint really well. I used good primer and color match paint from an auto body supply store and they look decent. not a perfect match but it looks alot better then rust holes LOL . I then had the truck krown oil sprayed and had them really get the inside of the cab corners and rockers coated well. So far 9 months and they still look great, no bubbling.

 

Oh and one more thing, if you have a power moonroof like mine you should pull the drain tubes out of the inside of the cab corner and route them out of the access holes so the water drips on the ground and not inside your cab corner.

 

So this is rust that is only forming on the outside under the paint? I thought this was coming from the inside and now creeping out. I have no moonroof and the 3rd brakelight has had a new seal around it for almost a year.

 

I sanded the rust off mine and panted the bottom and a few inches up with a bed liner paint. Its a short enough area that nobody has even noticed.

 

It is rusting from the inside out. Will need to be cut out and replaced. You can wait for a little while until it get's worse. My 2006 looked like that for 2-3 years before it broke through. Had it cut out and replaced. It was $450 from a small restoration shop...a lot of body shops won't touch rust or if they do they won't guarantee it not coming back.

 

That looks really good. I definitely want to do something like that.

 

It does absolutely nothing to protect it from rusting from the inside out, which is how cab corners go. You need to remove as much of the rust as possible and then oil the crap out of your rocker panels. I have sprayed the insides of mine with Fluid Film since new , and there isn't a spec of rust. Also, I live in Michigan where the roads are salted heavily. I spend HOURS every fall re-oiling the bottom of my truck.

 

Worst case you can buy aftermarket cab corners and install them by cutting out the current rusted area.

There are sellers that make a plastic cover to go over the rust but that doesn't really solve the problem.