The first step to any repair is diagnosis.. you need to figure out WHAT is wrong with them first, unless you're rolling in so much bank that throwing parts at it until you get it right isn't an issue.
So, you start by checking fuses and relays. If those are good, then you move to the switch, and test it for continuity (easiest way to do this is with a DMM). If that checks out, disconnect the plug for the mirror heater, and see if you have power there. If not, you probably have a break in the wire somewhere. If you have power and an uninterrupted ground, then the heater itself is the likely problem, and will need to be replaced.
It's a Peterbilt 379
The turn signal flasher relay switch, on a Peterbilt 379, is located under the dashboard. The relay simply plugs in.
2007
First indication is model year. The 379 replaced the 359 in production in 1987, and was manufactured until 2007.
How do you ajust the door on a 379 pete
1987
Peterbilt 379
Dana-Spicer
No
where is the defrost actuators located
wish i knew have the same prob on my 93 377 peterbilt
what is the freon capacity of a 379 pete with sleerer