I was wondering if there was a way to wire a magnet mount antenna through the cab brake light

 

You can but you run the risk of it leaking into the cab.

 

ok, where would the cable come out in the cab?

 

^ On the other side of the brake light...

 

I used to run mine down the door sill and up the C pillar in the back and out the small door in the back of my SCAB when I had it. My brake light leaked when it rained on my 04 until I got it fixed and I didnt want to risk it leaking again because it was about 40 bucks for a new light.

 

Do it right - drill a hole in the roof ,run the coax down the rear quarter panel. Done properly they won't leak. Mag mounts are not very good .