My 95 sc400's radio skips when i turn it on it like as a repeat of no sound constantly, i think its the amp.
If I bought an aftermarket amp like 500w or something for future proofing, could I just plug and rca to 3.5mm jack into it so I can use my phone for audio. Then plug the amp into the existing speakers?

 

the simple answer is yes... seek the advice advice from your stereo shop... my oppinion if you are not looking to run a deck (head unit is running a 7 band equlizer with dual input and 3 way rca output

 

The only issue I see is the very low input voltage from the output of the phone, likely less than 1 volt. Even an entry level aftermarket HU will have 2-2.5 volts. Will it work...yes, just not sure how much that low input voltage will effect it.