I am going to assume you have checked continuity on all the fuses involved, sometimes they can look good but still be dead.

1. Those switches have a tendency to fail when little plastic retainers inside break. It can be fixed if you are willing to fiddle around with it for 20 minutes. I fixed mine by replacing the broken bits with some wire that I ran through a couple of holes I drilled in order to keep the switch toggle in place. This has happened on all the Volvos I have had of that vintage.

2. Check the vacuum lines, diapragms and fittings between the actuator (big round black thing behind the throttle linkage) and the vacuum dump switch on the brake pedal lever (under the dash). It is usually a bad hose connection or diapragm, I have never seen a bad control amp although that doesn't mean rule it out if everything else is OK. The stalk switches might be the culprit but again, I haven't seen a bad one. Check all the connectors.

3. Am I seeing a trend here with column wiring problems? You may need to pull the steering wheel airbag (make sure you disconnect the battery first, you don't want to risk popping the thing) and check the horn wiring and contacts, you can check the switch continuities while you are in there.

4. Are you trying to operate the thing with the radio on or off. If I remember correctly (not a sure thing) it will only work with the radio ON. Check the various connectors at the switch and the antenna motor.

Hope this helps get you started.
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Bob Kraushaar '94 945T, '88 240, '84 242Ti, '88 300 TE, '89 560 SL, '68 Shelby GT-500 KR

I am not that experienced, but will give it a try for you.
1. Both sides of the light circuit are usually the same, just a switch to complete the circuit ,usually ground. IF one side does not work it should be the switch, connection at the switch, or the wiring has an open.
2. Could be lots of things. I would check the fuse and all the connections at the unit in the engine compartment.
3. The horn buttons on the steering wheel( at least on my 940) can be ornery.
You may have to replace the contact buttons on the wheel, or remove and try to insure that the copper spring piece is able to contact the contact point for the wire connection. They pry right off, after you remove the negitive ground cable.
4.Could also be a fuse or bad connection/wire.
Good luck!
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'94 940 150k, '86 240 170k, '72 142 KIA, '70 144 KIA, '69 144 RIP