The Dash DVD player feeds the switch box with RCA cables. Some builds we split those off and fed the TV as well. (So TV needs to be on AV1, switch box has no effect). If yours are not DIRECTLY connected to the TV (look at the back of TV for red/white/yellow cables), then the RCA cables feed the switch box and THEN the switch box converts everything to coax cable.
Soooo, with the switch box on DVD, that is feeding the TV with coax, so the TV needs to be changed to "cable" and then you need to scan for channels. With the dvd player going it should find channel 3 or channel 4.
Again, my recommendation is to split the cables and connect those cables directly to AV1 on the back of the TV. 1. It is a better picture that way and then when the DVD is playing just change TV INPUT from "AIR" to AV1.
PLEASE do not send questions via Private Message, my inbox fills up too quickly. Post questions in the forum as a new thread so that other users can benefit from the answers. Use Private Messages for sensitive information only (VIN, Contact info or things you do not want the public to see).