Cheap sub unit for older head units?

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I'm trying to work out how to get half-decent bass out of the Sony r/c head that I've acquired for the syncro (see the "period radio" topic https://club8090.co.uk/forum/viewtopic.p ... 3&start=15" onclick="window.open(this.href);return false;) but it's only got 4 channel output (but oddly appears to have a 3.2mm aux. jack input on the back)

So I was idly looking at cheap sub units on the 'bay that could be hidden under the rear bench and found that the Audi A6 avant (B5) units could be just the ticket, but have a 10-wire plug Two look to be ign. switched live and earth (red/white and brown, quite heavy gauge) so does that mean that the other 8 are for the 4 door speakers i.e the sub has a 4-channel crossover built in?

They look like the familiar channel colours (white, green, purple, grey) Anyone know for sure? These subs can be had for as little as £10 and they do give good bass