If you want to do this, then you will need a 1/8" (3.5mm) mono jack to a open ended wire connection.
The 1/8" mono jack can be found here:
1/8 Mono Phone Plug (2-Pack) : Phone Plugs | RadioShack.com
It is identical to a headphone plug, but instead of three rings (tip, ring, sleeve), it only has two ring - tip and sleeve.
You will need to solder to get this to work properly, but you will want a piece of wire that reaches from your speakers to your A/V receiver. Not speaker wire, but a light gauge wire like a 20 gauge between the receiver and the speaker.
Then, you would solid the two conductors to the plug. One wire to the tip, the other to the sleeve.
Alternatively, if you can find it, pick up a 1/8" mono to 1/8" mono cable that is long enough to reach.
Connect one end to these two screw down terminals:
Then the other end plugs directly into a 12v trigger on your Denon.
You will need to use TWO 12v trigger ports on the Denon I suppose, or perhaps wire them in series or parallel? Not strong on that part.
Here's a place that sells 10' versions:
10 Foot 3.5mm Mono Cable, Male - Male
You just cut one end off, strip the wire back, and you will have two wires underneath that you can use.
Or maybe one like this and you can cut it in half:
25 Foot 3 5mm Mono Plug to 3 5mm Mono Plug Audio Cable DVD LED LCD TV iPod Zune | eBay
Since the Denon has two 12v outputs, you should be able to have them both work identically if you aren't already using one.