JMcG-
The blue wire is already there, that is, under the dash and at the hitch connector.
I believe these two adapters fit the Freightliner trailer electric brake controller connector. If you use them, you leave the connector intact, so you or the next owner can install a controller by "plug and play." I prefer this to cutting and splicing the factory wiring.
Tekonsha 3027-P adapter
eBay Freightliner adapter
If your Ventana is on a Spartan chassis, let us know. There may be a similar adapter available.