I have a 1998 Volvo S70. The rear brake lights don't work. I have checked the fuses, changed bulbs, and checked the trees which are fine. What could it be!

There are several possible reasons. Obvious ones might be checking for fuses that are burned out. Second would be making sure you are getting good connections to your brake lamps (all 3). Third, check the brake light switch at the pedal. And last, Volvo Part Number 9442303 is a brake lamp failure relay that can go bad. It's not inexpensive, but probably more than likely the issue. One way to check is to find out if your cruise control works. If it does not, it is likely the relay. It is grey and flat on all sides. Earlier model Volvos had round red ones. Check the location in the service manual.

I would Double Check, High/Low LEFT/RIGHT/BRAKES/ Everything...

Fuses are a good bet. Otherwise might be a bad ground or loose wiring to bulb.

http://www.autozone.com/autozone/parts/Duralast-Stoplight-Switch/2001-Volvo-V70/_/N-jetkaZ9cias?itemIdentifier=365252_0_0_

check the fues or bulb

Check the bulbs first. Next check the wiring harness and then the switches. If not the bulbs it will be a bit of tedious diagnosing. It's the only way with major lighting malfunction.

if you managed the front ok, the rears will be a walk in the park good luck steve

None---It's ECU controlled. But if it bothers you because it's annoying, then you could reach into the dash behind the bulb with a 12 inch slotted thin screwdriver, and "fix it" permanently with upward pressure on the bulb! :)

Easiest way would be to go back to where you bought it and make sure the bulb is installed that you bought....and show them. Ask if you can get a new bulb and change it in front of him. Call the dealership and talk to parts, ask if, indeed the hi and lo are interchangeable.

Brake light switch may be bad or mis-adjusted.

Bret