I am doing an interior LED emergency light bar install for a firefighter on an '02 blazer across the visor area and the small screws I tried to use on the headliner won't hold them in place, & I was thinking of using a little longer self taping screws if the roof has 2 layers of metal so I won't go through the outer roof...does anyone know the answer to this?

 

No, there is only one layer of sheet metal on the roof. The only place where it might be safe to use screws is where there are support cross-beams, and even there you need to be careful that your screws are not too long. I'm pretty sure that your client will not be happy if you poke holes in his roof.

The other thing you could try is using epoxy or a very strong flexible adhesive to attach a piece of MDF to the roof sheet metal under the headliner in the area where you need to mount the lights. We used this method quite a bit back when I was installing custom video systems in vehicles.

 

I know a guy who used his sun visor mount holes to mount similar lights on a Ford excursion

 

just make some kind of "bracket" that goes on the back side of the headliner, just take a piece of 1/8th inch aluminum or something that is thin and flat, put it on the back side of headliner, and screw into it, will hold them plenty good