I need a wiring diagram to connect an antenna, TV, VCR and DVD. I have an RF modulator and a 2 input coaxial TV switch.

You first have to indicate what you want which product to do.
Is the DVD a player or a recorder-player?
Do you want the video recorder being capable recording from the DVD?
Don't you want the TV to be connected to the DVD directly by cable (better picture quality).
Etc. etc. etc.. . You have to be more precise !

:)

I am guessing VCR and DVD as one unit. Antenna in to DVD/VCR, out of DVD/VCR to Modulator, modulator likely puts out channel 3 or 4, then connect to TV. You don't need the switch.
If you wanted to record one channel and watch another, then have antenna signal go into a 2 way splitter, out of splitter to DVD/VCR, through modulator, then to 1 of the 2 inputs of A/B switch. The other output of splitter will connect to other A/B input. Common of A/B switch goes to TV.
Then once recording on TV has started, move switch to second position, then tune to the channel number from local station(s).
To check recording progress, select other position of switch and Tv to 3(or 4).
You don't mention Cable or Satellite.