mod for mono/poly?

[Isac Georg Jensen]

I just picked up a Korg Mono/Poly. I really like it (especially
the filter).  What i REALLY would love, is to feed the filter an 
external signal. I don't think there should be much to it, but 
before i open the beast, and begin to solder, could anyone please
tell me how to do this the right way ? (I have no experience in
modifying synths !) 
Any comments/suggestions will be very welcome. Thanks.

I.G.Jensen