Be sure you get the right one. My '85 123 takes the toothed mast. Send the mast up half way and unscrew the chrome nut on the cap assembly that is attached to the fender. Now when you put the mast up the whole way it will just eject out. Have someone help you because you want to catch the mast so it doesn't fall and chip your paint and also as it comes out note which way the teeth go (assuming yours has them). Putting in a new mast is easy. Have your helper push the button to retract the mast while you feed the end of the new mast in. Once it catches it will pull the plastic spine in. Stop it before it retracts the whole way and tighten the chrome nut (not too tight or it will break. Don't ask me how I know). Now retract it the rest of the way and you are done.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold