You are quite lucky that no one has cut the factory wiring for the stereo system. To avoid having to cut any wires you can get this from Crutchfield:
Model number: 120701822
http://www.crutchfield.com/p_120701822/Receiver-Wire-Harness.html?showAll=N&search=120701822&avf=N&skipvs=T
That wire harness will allow you to install almost any receiver. However, there are also some Blaupunkt DIN to RCA adaptors you can find on Ebay if you want to maintain your Reno, but are looking for clean power out of an external amplifier.