Add an Amp Kit - EbayDo you have a specific example? The OEM stereos in these cars are junk, I'd like to see an example of one of the kits that you're proposing in order to let you know if it's worth your time/money.
That is just a line output converter with an OEM harness. There are better line output converters (LOC), if you are willing to tap into the OEM harness. If you don't want to bother doing that, then the link you posted should work just fine. You'll still need to run the RCA's and all of the amp wiring.