Thanks for the reply.
I'll go for the non powered one then, does seem the easier to install and the sound quality is never going to be amazing with the ford cassette head unit, but I like analogue sound so I'm keeping it. Just want to be able to turn it up without distorting. speaker upgrade to come as well
Oh and does it matter where I put the converter. Is it better near the head unit or can I splice into the rear speakers in the boot? - where the sub will be, so a lot easier.
I would just hook it up to to one of the rear speakers in the boot, that way you are keeping the wiring from the convertor to sub(amp) short
As the sub is "mono" or single-channel, you can just use one converter, taken from one channel(side) or rear speaker, this will simplfy installation and may reduse the chance of crasstalk/ eath loops, hum or buzzing
interference/ hum/ buzzing can be a problem with these converters if you take the power for the sub(amp) straight from the battery (via an inline fuse) you could fit an on/off switch for the amp near the radio, that way if there is any hum from the sub you could switch it off when you are not listening to music, or even when you are listening the the radio/cass quietly (it would also save electrical power)
if hum/ buzz is a problem you could adjust the settings on the sub amp, or fit a (passive) ground loop isolator between the converter and sub-amp