I don't know if my RCA outs are broken or what?

I've got a Pioneer headunit that has RCA out's only, it's called a DEX-P9 (it's not the same as the English DEX-P9... along the same lines, but an older model from Japan.) Anyway, it's to sell on ebay, so I tested it, and what I found is that the RCA's only work when I ground them. Say I'm running something off one set of RCA's, there will be nothing until I touch one of the other RCA plugs onto the grounded body of the unit, then they all work fine.

I initially presumed it was a faulty unit, but I have just got another DEX-P9 and it is the same, so now I'm confused!