No activity in the sight glass usually means either nothing (dry - no oil or coolant) in the system = catastrophic failure (leaking AC line, compressor, evaporator, etc) rather than just a simple minor R134 or R12 leak OR the compressor clutch is not engaging.
SO - check the AC fuse, AC relayand Compressor clutch wire too.
With the engine "off" but ignition on, can you hear the electric AC clutch "click on" when you hit the *snowflake button on the AC control? If there is no click, then suspect fuses, relays, even the HVAC control (little yellow light still come on in the switch?). There is another simple thing to check - get under the car and make sure the single BLACK wire connection from the compressor to the spade connection (it's about 8" long) has not been disconnected. If your car was "serviced" recently, there is a chance this was disconnected - it's really easy to do and forget because it is a rather thin wire. It would not be the first time this has happened to owners either.
If all this works OK, then start looking for compressor leaks (the oily streaks etc).
If you've lost the R12 (92 cars) or R134 (93+ cars) then you need to do the leak tests etc. Most countries, you cannot get R12 anymore, but check Greimann's post
http://www.968forums.com/index.php?showtopic=905 on HC coolant - I'm waiting for my kit to refill a system but they are also on eBay for less $$. Get some leak detector (just dye in the coolant fill) if you suspect a leak in the system. A good AC check at your serviceman should include a liquid nitrogen charge / check (much better for the environment) to find the leak.
For our 85.5 Porsche 944NA, I just changed a compressor ($200 on eBay with 1 year warranty from a 6000+ feedback seller) and a line. I also changed the receiver dryer ($30 - cheap). A good idea to get a LOT of new "O" rings while you are at it however. My total cost for the whole install re & re will be under $350 plus 8 hours labor removing and installing everything. My 968 Cab checked out fine with the Nitrogen test, but since it was a R12 fill, I cannot get anyone to recharge it, hence I will try the HC kit refill that Greimann suggests.
If the car blew ice cold 6 weeks ago and there is absolutely nothing now in the sight glass, that sounds like something more catastrophic / or faulty electrics rather than a gradual slow leak. I'd check out the electrics first - that should be easy 30 minutes max.
