I came across the same problem...I bought a DIN adapter that works with the stock amp, but didnt end up using it becuase I found out that my amp was fried. I dont think the adapter will work for you since you spliced the DIN cable. Did you actuly cut the connector off? There are more that just speaker wires...remote/acc/prw/ground so it would be hard to indetify and connect the speaker wires. I just ran new wiring form my aftermarket deck to the out put harness of the amp that leads to the speakers. There is a sticker behind the amp that idetifies each wire to each speaker (FL+/- FR+/- etc) I ran a total of 8 wires +/- for each speaker. You have to take apart the harness and free each individual wire. Its a matter of matching and connecting, heres the wire color code:
1.FL- light green w/dark green 2.FL+ light green w/violet,
3.FR+ light blue w/violet 4.FR- light blue w/dark green,
5.RL+ white w/violet 6.RL- white w/dark green,
7.RR+ tan w/violet 8.RR- tan/dark green.
This is just using the amp in the deck to power the speakers not for an aftermarket amp, I have an aftermarket amp but its just for my subs. Hopefully this helps and you can save some $$. Or you could use a female DIN connector heres the link, it will work but only if you didnt cut the DIN connector off...
http://forums.viperclub.org/virginia-maryland/624799-any-head-unit-install-gurus.html