Welcome, Paul. IIRC, 99's have a electrically operated SC clutch, the Motronic (ECU) turns it on and off as needed. At idle, the clutch is not engaged. With the transmission in park, you can move the throttle by hand to increase the RPM to ~2000 and the clutch should engage (you'll see the inside part of the pully start spinning).If it doesn't, the next step is to determine if the Motronic is trying to engage it. There is a two-pin barrel-shaped connector on the front of the engine near the SC, you can measure the voltage here to confirm/deny. It's a common failure for the butterfly valve atop the air filter to fail, I think the Motronic refuses to engage the SC if it smells a rat there. A quick sanity test of the butterfly valve is to disconnnect the hose (the one with the spiral wire) so you can see it, then turn the key off and back on a few times. You should see some slight motion of the butterfly. Inspect the wiring carefully to the butterfly valve since it gets stressed when the air filter is replaced.