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The E28 528e ECU is located above the glove box. Open the glove box, and for better access, also loosen the two straps from which the glove box hangs. Where the straps attach to the glove box, the black plastic fasteners for the straps can be pushed out by hand to some extent, and a flat-head screwdriver can help coax them out the rest of the way.
The wiring harness that feeds into the ECU ends at a massive plug that is held into position by a spring-loaded clip and a stay. The stay is attached to the ECU with two tiny screws that are too small for a normal screwdriver. A tiny flat-head screwdriver is needed to loosen or remove the stay so that the plug can be freed.
The plug and cable obscure two of the bolts that attach the ECU to the metal plate above it. Two other, non-obscure bolts also have to be removed. They are all 10 mm bolts, and they point up.
When working on this sort of thing, it tends to be a good idea to have the battery undone, but when you reconnect it you’ll also have to re-enter the security code for the BMW factory-installed sound system, if you have one.
Dropping the ECU can kill it, so it might be a good idea to have some sort of padded-bottom box to put it into when you take it out of the car, assuming it’s still good or likely to be.
Installation is basically the same as removal, in reverse … no special subtleties of which I’m aware.