What kind of TV?
Some newer TVs with digital tuners may have a digital output that you could use when you're using the tuner.
Otherwise in most cases you would just use the RCA analog audio output from the TV (hopefully it has one, very cheap TVs tend to not have that feature)
If the TV has a stereo analog output, you just need a cable to connect to your speakers.
https://www.cablesforless.com/index.asp?PageAction=VIEWPROD&ProdID=13735
You'd run to the line-in on the control pod of the 680s.
I'm not sure if you can apply surround processing to the line-in input on the 680s. If you can, you could try dolby prologic II to get all speakers to play. Otherwise they will keep the signal pure and just give output to the FL, FR, and sub.