How to install 6 inch speakers in Rear of 240D 83

[Carrameow]

The existing cut-outs are for 4 inch speakers and have plastic/ABS speaker wells underneath them. My new AIWA 6" speakers dont drop in fully into these wells, they need about two more inches of depth because of the 2" diameter difference. Can I cut the rear deck with a jigsaw without ruining it? Any ideas? My other idea is to put 2 inch tall 6" rims under the speakers on the deck.
BTW Found out the hard way : the new 4 separate output stereos have the outputs out-of-phase--you can't tie into the existing 4 speaker 2 wire common ground wiring harness--must rewire the speakers, or at least two of them. WHATS THE BEST WAY TO ROUTE WIRES TO REAR OF CAR wWITHOUT RUNNING THE WIRES UNDERNEATH (just joking) without ruining the interior?