I put an aftermarket seat in my 123 in about an hour. It's just like the factory setup but doesn't have the three point seat belts, only lap belts. Pretty much all the holes you need are already there so there's not much you need to do other than screw it in. If you want the true factory setup then you'll need the panels, retractors, etc. for the shoulder belts and those parts you'll need to scavange or pay pretty big $$ at the dealer to get. My aftermarket setup works great for accasional use and the whole kit cost about $450. You can find them at any of the places that sell replacemnt seat covers. The 124 seat looks like it's a bit more complicated so may not be as easy to install. They are definitely NOT interchangable with the 123.

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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