Two Head Units?

I think that's not the correct model number for the Aiwa head unit because when I typed that into google as well as Crutchfield I got basically zero results.  I think the sony head unit can use an auxilliary input as long as you have the special adapter which unfortunately is like $100.  But what you'd do is hook up an RCA cable from the Aiwa's preamp outputs to the auxilliary input adapters RCA inputs, and then you'd choose the "AUX" source on the sony head unit.

Unless you use external amplifiers, the sony's built-in amplifier needs to be functioning properly.  I wasn't sure if that's what you meant when you said "bad radio" or if you really meant just the radio reception is bad.

Of course if the Aiwa is one of the many Aiwas out there that already has an auxilliary input built in to it then you'd just use that, and hook up the sony's preamp outptus to the aiwa's auxilliary input.  You may need a y-adapter to go from the RCA-type cable of the sony to the 1/8" headphones type plug of the aiwa.

Finally, both units are going to need to have power (switched and constant) as well as ground connections made to them.  Depending on what size fuse is on the vehicle's wiring for the radio harness, you may or may not need to run a new fused wire directly to the positive battery terminal.  Just try to hook up both units to the single set of wires in the vehicle's radio wiring and if you blow a fuse when you turn it up real loud then you'll know you need to get a different source of power (such as the battery terminal).