First off the quit making the 780 in 1990. What do you mean brake relay? Brake light relay/integrity sensor? Need more information
Please go to this URL. I think 780 has the same relays and locations: http://www.davebarton.com/pdf/relay_87_700.pdf Igor.
On a 1990 Volvo 780, the bulb failure relay is typically located behind the dashboard, near the driver's side. It is often found in the relay panel, which is accessible by removing the cover beneath the steering column. If you have trouble locating it, consulting the vehicle’s service manual can provide specific details and diagrams for your model.
Assuming you're referring to a Volvo VNL 780 truck, with the double bunk sleeper, 12'6".
You'd have to contact Volvo customer service for this. IIRC, there were a couple different options for the 730 and 780.
with a fridge
96 inches.
Try seaking seal
Volvo dealership. Have fun with that... if you've never taken off a Volvo starter, you could be in for a lot of excitement.
16"
15,000 to 18,000 lbs., depending on options.
You do not need any special tools to change out the auxiliary fan switch on a 1989 Volvo 780. The wrenches you normally use are sufficient.
On the right side of the throttle pedal