Well you may be in luck. When I got my '97 S320 several months ago, I noticed what sounds like the same thing. Then I found the instruction manual. Mine is a either a
UN700/SW/SWL or a UN3000/SW/SWL by K40 Electronics Elgin IL 800-323-5608.
I understand this is a high dollar system well over $1,000.

The outer knob is City (left) Off (center) and Highway (right). The inner knob is volume.
If you have three indicator lights, then left is front alert red, center is City or Highway indicator Green = Highway, Yellow = City, Right is rear alert.

Having said the above, mine lights up but is flaky. Sometimes alert continues to beep ad nauseum. I also don't think that, in my case, a now 10 year old unit is up to today's technology. I turned it off, and continue to use my Valentine 400, which has saved me numerous times.

If you want a pdf of the manual, really just a two sides brochure, send me a private msg with your e-mail address and I will scan and send it to you.

Steve