My blower has been dead for nearly a year so decided to swap it out and repair the air-recirculation box as it was frozen in a half open position. Here's a brief right-up.

Blower removal on a 1989 928 S4 right-hand drive.
1) Park car with enough space to fully open passenger door. You'll need all the room you can get whilst knelt down doing this work.
2) Disconnect battery.
3) Remove passenger carpet and fuse panel wooden cover.
4) Remove parcel shelf and remove it from the car.
5) Undo glove-box so you can move it in and out; you don't have to take it completely.
6) Remove an electrical units and plugs that are in the way of access to the bulkhead/air-recirculation box.
7) loosen the 4 bolts and 2 screws holding the fuse panel retaining brackets and gently pull the fuse panel forwards to a near flat position. Don't put any strain on the wires; you just want it out the way for access.
8) Move the fuse panel brackets away from the bolts that also hold the air-recirculation box. This can be tricky and takes patience.
9) Undo all the nuts holding the air-recirculation box.
10) Remove metal strengthening bar at the bulkhead.
11) Now you need to loosen the goo/putty that seals the air-recirculation box to the bulkhead. This where having the glovebox loose helps. Start at the front under the glovebox using a small chisel and long flathead screwdriver. This takes time and care not to damage the air-recirculation box. Keep at it and it will slowly pull away from the goo/putty seal.
12) Now, go and open your bonnet and remove the blower. Take off plastic cover over blower/window wiper linkage era; disconnect and remove rubber cover between blower and HAVC box; remove blower retaining screws.
13) Go back under to the recirculation box in the car and pull it fully away from the bulkhead and blower. It will take some jiggling as it is a tight fit. Remember to disconnect the blue vacuum line at the back; mine was loose anyway.
14) Now you can pull the blower down through the hole left behind. Jiggle the blower out and then disconnect the wiring to the blower.
15) Congrats; you got the blower out! Now have a break and some food/drink as it might well be 2-3hrs you've been working!
16) Now do a clean-up behind the fuse panel and in the bulkhead/HAVC box inside the bulhead; there will be lots of old leaf debri/mess that is best remove. Use a hoover and screwdriver to loosen crap; go careful with that screw driver inside the HAVC box! Finish with a warm clean rag.
17) Now get some clear waxoil and a brush and paint in behind where the blower goes. This area is prone to damp.
18) Now, and if you have a blower replacement, reinstallation is a reverse of the procedure. Good luck finding a new blower/impeller nowadays! Good used items seem the only way currently. Clean blower casing etc up as best as you can.
19) This process can be arduous as you're working in a confined area and have lots of electrical stuff exposed; be patient. If get frustrated take a break and come back later. It will all go back together eventually!
20) If you're repairing the air-recirculation box/actuator like I did here's some tips.
21) My flap and actuator were seized so I cut the actuator arm off where it connectors to the metal rod which opens the recirculation gate. Removed the circlips holding the rod in place and then using grips worked the rod out. Replacing the actuator is easy and self-explanatory. Just remember to position the vacuum pipe off the actuator back in the right place for the pipe in the bulkhead upon reassembly. When reassembling apply liberal grease on the metal rod. Clean the recirculation box up well and lubricate the gate hinges. Finally reapply putty/seal to the recirculation box for reinstall. It's available from Porsche.
22) When finished enjoy you improve blower/heating system!

ps: The rod that the vacuum actuator is attached too is held in place with 2 circlips; 1 at either end. They're a tight fit and fiddly to refit after removal. Be careful not to loose one with it pinging off someone! I speak on experience. Luckily circlips are plentiful!

Hope this helps other 928 owners as there's little info on this work.
cheers, Dave.
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Pictures say it all.....