Shouldn't be RF modem. I used to do this with a Maxtrac. I don't think I had to do anything with it. If you do have to do anything, I think it's on the first page, "accessory connector" (F4->F2? I haven't ever used Spectra RSS). Just grab a plug/accessory kit for the 15 pin connector, wire up an audio cable to the audio out and GND connections, and you're laughin'.
If you mean, 'show the system activity with Unitrunker', sure, if (as others have said) baseband audio is available.
If you mean, 'Have Unitrunker control the radio to LISTEN to the trunked system', I don't think so. The radio isn't designed for computer control, I believe.