dimmer question on old house : third wire?

Hello, I have a question about installing a dimmer in a room. My house is quite old.
I am going to be replacing an older dimmer that isnt working well. This is how it is installed now: The actual dimmer itself, has only 2 wires. One wire goes to a thick black wire and is connected with those plastic wire connectors.
The second wire from the dimmer goes to a thick red wire also connected with those plastic wire "caps".
I realize that its pretty simple to connect a new dimmer, however, I noticed that new dimmers have 3 wires in them. Where should I connect the third wire? this is a ground i imagine. If there is no room to connect the ground, does it matter to have it at all? I mean the old dimmer had only 2 wires, no ground. THANKS!
KOS
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Check that you did not buy a '3 way' dimmer - it is much more expensive than a regular '2 way' dimmer and would have 3 wire leads and all 3 would be required for that application..
I'd suggest reading the instructions that came with the dimmer to verify that the third wire (which would be green in color if it is a ground) is in fact a ground.

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