Assuming it's the same 4" speaker that's in my 87 240 then here's the deal. Many aftermarket 4" speakers will fit in the hole and the depth isn't an issue. You need to by an adapter plate so you can use your stock grill, your car stereo shop will have these or be able to get them for you. The problem here is that many aftermarket 4" speakers are to tall, the distance from between the speaker and the grill, not the speaker to the window. This is easy to check. When you go to the shop have them open the package with the plates, bring a grill with you, and set the speakers inside and try to put the grill on. On have alpine speakers in my doors and they're very borderline. So close in fact that I mounted the speaker behind the plate to gain the half a millimeter. I beleive MB Quarts fit but they're pricey. Infinitys do not fit. Alpines do fit and try some others. If the door is different then hopefully somebody will find this useful.

Ken
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Portland, OR 1987 240 dl wagon (not so rusty anymore) 'rustbucket'