Third break light will not work.

[Mr. Sinister]

I need some help w/ my third breaklight it has no voltage at the light ; I've checked the fuse and it is fine ,replaced just to be sure still nothing. I can't check the fuse box for volts ,my meters probes are too big.So I was thinking ther might be another fuse but I can't find it . Any help would be appreciated; thankyou.

[dingbat]

check wiring harness under truck and going into tailgate. You did a lot of mods back there and might have damaged wire. See if something looks obvious.
Is connection to new brakelight OK?