Need help upgrading from 8 to 10 speakers using OEM crossovers

My 2008 Golf Plus was an 8-speaker system from the factory, so the front doors have a woofer and tweeter with no dedicated crossover - the direct speaker outputs from the head unit go straight to the woofer and are looped through the speaker connector onwards to the tweeter.

I've added mid speakers to the front doors and I picked up a couple of OEM crossovers cheaply from my local specialist VW breaker. Wired up the left door, and the output is incredibly low; you can just about hear the woofer, but the mid and tweeter might as well not exist. I tested the crossover and it's definitely working on all outputs, and I also swapped in the other one, which exhibited exactly the same behaviour.

The speakers are all the correct 4 ohm impedance. All I can think at this point is that, because the 10-speaker systems all came with the separate power amp, the crossovers need to be fed with a higher-level input than the head unit alone can provide?

Secondly, I'm a bit confused by the wiring diagram. Why does the mid speaker have two positive inputs and the tweeter have two negatives? As things stand, I've got them wired so that the mid only has one positive (pin 5) and the tweeter only one negative (pin 10).

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