Why was Egon underneath Janine's desk?

Ghostbusters is unusual in that the studio commissioned two different screenplay novelisations which often leads to some weird canon clashes. In this particular instance, however they both agree. Egon was doing a wiring job on the alarm system connected to Janine's desk.

Spengler crawls out, having finished the wiring job. He gazes down at her vacantly, his lower lip jutting out, a sure sign that his tremendous brain is struggling to grapple with the real world. Nobody has ever bothered to explain the facts of life to Spengler; he worked them out for himself on a pocket calculator and vaguely suspects he came up with the wrong answer.

_{^{Milne Novelisation}}

and

Today, at least, he was doing something comprehensible, crawling around beneath her desk, connecting up an alarm system. Occasionally he would poke his head up, look around to get his bearings, and disappear below once more, his hands full of tools, a wire clamped in his mouth.

_{^{Mueller Novelisation}}

Interestingly, an earlier version of the screenplay states that he's installing a phone system, not an alarm system although it's conceivable that he's using the phone line as a carrier for the alarm.

RECEPTION AREA - DAY

A bored-looking red-headed young woman, JANINE MELNITZ, sits in a swivel chair behind the reception desk, putting another coat of red polish on her heavily lacquered nails. Spengler is on his hands and knees wiring up the telephone system.