The Bose speakers are going to be an impedance...usually 3ohm. If you are going to do a complete rewire then it would be a good idea to replace the speakers at the same time. You will have the entire car apart and with speakers that old they are due.

But you can do it without a rewire. You can turn the bose amps on and feed them a low level signal. Several companies make adaptors you could use also. Peripheral has a part available...model number SVHNI1. It is made for Nissans with Bose and does not require a rewire.

Personally I installed aftermarket radios with Bose systems hundreds of times with no problems using both adaptors or cut RCA cables. You can do that. But due to the age of the car I would put in new speakers and rewire the system. You will get the most of of the stereo that way.