Your right about the crossovers, each speaker output is routed separateley from the amp, so each front door has 3 pairs of cables and rear speakers 2 pairs, so you'll need to route tons of extra cabling through the door plugs, plus door skins off as that's only way to get to door cabling, big project when you can just use a 4 channel amp with existing cable.
I'm pretty sure there isn't a stand alone or after market loom from anyone for such a rare thing to retro fit, hell of a job to give a go.