Need Help- new horn install

[Huckleberry]

I have to replace the entire horn assembly on my 1978 GMC K15. Where is the safest place to drill in the firewall to run the horn button wiring?. Truck has no AC unit. thanks in advance D

 

[MarineOne]

Sneak it through one of the factory grommets by brake booster

 

[Huckleberry]

Thank you sir. I'll give it a try.