Here's a little trick I use quite often to see if parts are interchangeable between cars. I go to fcp groton's website, www.foreigncarpartsonline.com, and look up the part numbers for whatever parts from the cars I have available to me. So, in your case you'll want to look up a power door lock motor for a 85 740t, and an 84 240 whatever. If the part numbers are the same, you can swap em. Plus, you can see how much money you've saved yourself if it turns out that the trade is doable. Big fun!
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1981 244 glt