[Richard]

Stock radio used a single 10 ohm speaker. Using an 8-10 ohm speaker/speakers for testing will not hurt anything, even if output will handle 4 ohms. Volume will just not go as loud with the higher load speaker. If there is more than one speaker connection the output section has been changed and would bet it will handle 4 ohm speakers. The Aux input implies the preamp section has been changed. Any newer replacement/upgrade boards would be designed for automotive speakers which are commonly 4 ohms for many years now. Looking at the board is a good idea, would bet board is labeled where output wires connect to it. Would help with connections.