As losh above says they can be rebuilt. I personally spent a good deal of time on this.
Firstly if the aerial occasionally dosent lift when radio is turned on what that is is dry solder joints in aerial module. Especially noticed when cold..
Secondly the power aerial itself. If yours is genuine buy a genuine rope and mast a few still available on eBay. Remove unit from wheel arch.. Getting old one out is a pain you have to tap a jewlers type screwdriver between it and housing twist metal with pliers and withdraw. When replacing use white grease or similar on rope as well as carousel. This is important as it will time itself if out of step eventually and may take many weeks to really settle so lube is important. When working properly they finish with two taps on retraction and elevation and sound great.
The other thing you must pay attention too is on reverse side of unit reverse to carousel that is from memory if you open that you will notice that rope runs against a metal grommet which keeps the teeth of rope hard up against motor gear.. This grommet wears so play develops as rope runs against gear so it skips. People think rope teeth wear so it skips but it is usually this. The remedy is turn grommet around where it isn't worn.( they wear from people leaving radio on so every time car is started aerial rises unnecessarily. )
Also clean antenna contacts and apply NO OX ID grease or similar and try to get electrical resistance as close to 0.01 ohms as possible. Check also earth for unit is tight and clean. Remember too that original radio tuners did go off I had tuner replaced at overhaul. I also spliced antenna cable near radio and ran another earth on metal inside car to give OE type reception.
Hope this helps any questions ask.
Good luck.
PS.
I believe if aftermarket unit fitted aerial will not work when turning on. If aerial dosent rise when pressing aerial rise button probable aerial module dry joint issue. Obviously check fuse for aerial circuit first but remember around 15 years ago was to fit CD unit and apart from losing click was told I would have to make aerial rise and retract manually by pressing buttons on pinnacle and that turned me off so that's how I have original head unit.
