I have known cars that have the prepump inside the tank (as yours has)to run fine even when low on gas, but your symptom does sound like it could be caused by your prepump not running.

To check it: get a short piece of hose (garden hose or smaller) and a helper. You put one end of the hose into the gas fill hole, just far enough to get past the flapper so you have a clear shot to hear what is going on in there. Put the other end in your ear. (if you have a flapper in your ear, we don't want to know)

Have the helper turn the key to the "run" position (position II), but not as far as "start" (position III). If all the dash lights come on, but the engine doesn't crank, it's thr right position.

You should hear a buzz in your ear for about one second, then silence again. If so, the pre pump is running. If not, your mechanic is going to have to get back in your trunk and correct whatever he/she didn't do right the first time.

The main pump, under the car, also runs and buzzes during this priming sequence, so the hose is to make sure you are listening to the one in the tank.

Good luck and let us know what you find.
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Andy in St. Paul - '91 745 227K mi, '91 745 210K, '90 744 194K, all Rex-Regina; '86 745 GLE M46 230K; past 240s