The stock head unit has a wire the same as the aftermarket, which turns the antenna on. So hooking just that wire up will make it go to whatever preset height you have it at. So before changing head units I made sure it was set to full height, so now when the aftermarket head unit turns on, it goes to full height. Hope that makes sense...
Now, if you wish to adjust the height still, you'll need a switch with 3 positions. You would wire the centre position as nothing, wire the top position to the UP wire (WHITE WIRE) and wire the down position to the DOWN(LIGHT BLUE WIRE) wire. A spring loaded(so it reverts back to off by itself) rocker switch with a neutral centre position would work well. Just connect the centre pin of the switch to GROUND and connect the WHITE and LIGHT BLUE wires to either end of the switch. This should work, but if you don't understand, get a pro to do it
See schematic of VT/VX radio system
here.