If I understand corectly, you really want to be able to run the video fron your OTA antenna to the 990 and be able to feed that to the TV while your using the 990's AM/FM tuner to listen to a radio broadcast.
I don't think there's a straightforward way to do that as the 990 doesn't have coax conections. You'd have to find some way to get the video signal to component,composit or S-video in order to fed it to the 990.
If you have an old VCR around, you could use that to split and convert the signal from the coax connection, but it's not a very elegant solution.

There are two ways to watch video from an OTA TV signal while listening to either the radio or the TV audio.

Option 1 is to do what we've been describing in this thread. That is to use the TV's OTA tuner. In that scenario, the video signal doesn't need to go anywhere outside the TV. All you need to do is connect an audio signal from the TV to the 990. You could use the TAPE input (rename it to TV if you like) for that audio signal. Once you do that, you go to the station you want on the TV and then switch between the TAPE and TUNER inputs to use the different audio sources.

The second option, which will involve having both video and audio pass through the TV, requires that you use an OTA tuner that is not in the TV. The simplest choice here is the VCR, as KOYAAN suggests. You can connect that VCR to the 990 (we'll use VIDEO3 as an example input), and then use the 990's monitor output to go to the TV. In order to view the VCR's video signal while listening to either the TV broadcast or the tuner, you will need to switch to VIDEO3 first. This will establish it as the 990's video input. After that, you can toggle between TUNER and VIDEO3 to change audio signals, and because the TUNER doesn't have an associated video input the 990 will leave its video switching set to VIDEO3 the whole time.

From an audio and video standpoint, the two solutions should be effectively identical. If you already have a VCR or other device in the signal path that has an OTA TV tuner, you could use the second approach without having to add an audio connection between the TV and the 990. On the other hand, if you are using a cable or satellite receiver with built-in DVR for your TV viewing and time-shifting most of the time, you may not have anything with an OTA tuner except for the TV, and adding a stereo analog cable may be easier than adding an old VCR.

One more wrench to throw into the works: you've got thirteen months to use this arrangement before OTA analog TV broadcasts cease (I remember the date as the day before our daughter's fourth birthday). After that, you will need a digital OTA tuner of some sort (either built into the TV or in a standalone box like your Zenith HDV420) to get broadcast TV, and there's a good chance that the synchronization with your local radio station may no longer work so well.