Make a hole you can pop this into. Plan the hole location to avoid any internal bracing.
Find the wires running to the tens. Move them to the top post pair.
Run new wires from the bottom post pair to the tens.
Install into the cab.
Leave the jumpers in place for normal use. Remove the jumpers when you want to use only the cab; connect the external amp to the bottom post pair.
There's a more elegant way to do it that might give you external access to the cute little horn, too, but you've asked a simple question so I figured you're probably looking for a simple solution.
(Warranty = VOID.) ;-)