The plug Mark's talking about will connect the speakers back to the head unit rather than the Bose amp, allowing you to use another head unit without an amp.
The plug for the head unit is standard, except that there is an extra connector as part of the moulding in addition to the standard DIN plugs. You'll need a DIN adaptor lead to plug a head unit into the DIN plugs on the loom, and you'll also need to swap over the two +12v leads (most adaptors have a couple of bullet connectors for this.
If you want, in addition, to add another amp, I would suggest taking out all the Bose wiring and amp and selling it (there is a market for this stuff) and starting from scratch wiring up the amp. You could probably hack about with the wiring under the parcel shelf to drive another amp, and there is a HOWTO document on here describing this process, but you'll have to run preamp signals from the head unit to the amp anyway, and probably a beefier feed to supply the amp with 12 volts, so you may just as well run the whole wiring through afresh IMHO.
Kevin