hi. sorry cant tell you wiring locations but i have a audi 80 with abk engine. Has the new switch been earthed itself? maybe bad earth for idle valve. i know that cleaning the idle valve can help if its very dirty and sticking , my car has been idleing badly and i just cleaned the idle control valve today and its better but haven't taken it for a long run yet. the valve should have an internal resistance of 2-10 ohms, 2 being cold and get higher when hot. i have tested mine a few times but always comes up at 5ohms when hot or cold. i am guessing the wiring one must come from ecu and other a group earth.
i think the idle conrol valve is fused at number 27 in fuse box.
i have had problems with fuse contacts being dirty and not operating properly so may be worth a look? sorry cant help more, it may be a case of following the wiring maybe finding the problem. have you tried the diagnostics to see if there is something wrong?