As you probably know the standard 172 head unit has no pre outs so you cant connect an amplifier unless it has HI-level inputs(speakers inputs). just thought this speaker-pre out converter on ebay may be of interest to some of you.with amps with no hi level input
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=803102619http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=803102619