Like a lot of other Blazers mine started rusting in front of the rear wheels. I bought spray rust reformer and intended to drill in the inner panel across from the rust. There was no way I was going to try to get to the rust thru the interior panels. When I looked to find a good spot to drill, I saw a plastic plug, covering a hole as big as a quarter. I popped it off, vacuumed and then sprayed exactly where I needed to. If I had known about this plug I would have sprayed my 2004 Blazer a few years ago.
Look for the plug about 8" in front of the wheel well. Get a good spray rust reformer and watch out for your lungs!