Trying to Connect Comcast Digital Cable Box and DVD Player to an older TV with only a coax cable connection?

It works fine with cable from wall to Comcast Box and comcast box to television, via an RF modulator. I tried connecting the DVD player to the cable's auxiliary input box, but I cannot get the unit to recognize the input device... any input device for that matter, whether a VCR, a Nintendo, or any DVD player I connect. Thanks!