I tapped in to the tape leads and have a really high level on the L channel and a normal level on the R channel. I haven't put in an attenuator yet. Any thoughts as to what's up?

Even with my beginner's soldering skills I was able to cobble up a workable DIN-to-RCA adapter. I got the DIN plug from Digi-Key (very cheap, I bought a package of 10) and the goal was to connect the Blaupunkt head unit in my daughter's 940 to a 2x40W Volvo amp salvaged from a late 80's 760, to drive the Infinity 6x9's in the rear deck.

The Volvo amp was something like $15 at the PnP junkyard and I pulled it with its mounting bracket to hang it in the trunk under the rear package shelf. Long RCA line-level cables from the head unit to my adapter harness and its female RCA's, then the DIN plug into the amp. The twin RCA cable contains a separate wire to turn the amp on...this is also spliced into the DIN adapter.

In short, it sounds very good. Even at moderate volume there is a notable improvement - sound is more "full" and seems less strained. And it can be made to get very loud...even generates the heavy bass that kids seem to think is the main feature of "music".

If you are retaining the Volvo head unit you can solder up an adapter for an aftermarket amp, but I would think your best path to better sound is to install more powerful Volvo amp(s) as suggested below, which should hook up with much less custom wiring hassle. Those amps are frequently on eBay at good prices. Quality aftermarket speakers would be the finishing touch, but watch for ones that fit Volvo doors - many don't.

Volvo uses a cable from the radio to the amp which turns it om/off, plus the audio connection is of a level that seems to pick up ignition noise. Crutchfield sells a cable to adapt a radio into the cable, bypassing the amp. I've tried to use the amps separately, but to no avail.
As for tapping in a CD,Ipod, I'd look in the area of the volume control, maybe you can find the switch (output of the pre amp)for the radio-cassette.
Tapping at the input to the tape amp needs an attenuator as the output from a tape head is about 1/100 the line output of the iPod/CD. Also tape heads need EQ, so the response will be bass heavy.
Good idea though, something I should do, but my 740 radios seem to have a chronic FM problem.
--
84 242Ti IPD bars&springs, 89 745 16v M46 IPD bars, 89 744 16v M46 IPD bars, 93 945 Turbo AW71, 91 245SE AW70 IPD bars, 93 245 CLassic M47

If your factory amp is mounted to the back of the lower radio/storage pocket then I'm pretty sure its a 2x15w. I've only seen the tiny 2x15 back there. If you pull the side trim off you might be able to use a mirror to read the printing on the amp, as I recall its written in giant white letters.

You could stay Volvo and get a more powerful amp out of a 740 fairly easily. I have a pair of 2x40watt amps from a 740GLE that I like with my CR-2170. They're very large though, you'll have to find a new place to mount them. There is a small 4x20watt amp in most 740s that is easy to mount, and the 2x25 isn't hard to find space for either. I think 780s came with even larger amps than the 2x40, and I know the FWD cars do as well.

Of course buying/making a DIN to RCA adapter and installing an aftermarket amp is also possible.
--

I have the TD-613 and didn't see a plug for the EQ on the back - maybe I was looking for the wrong thing though?