Easy Ext. Speaker Out?

Have a 100 watt Fender solid state amp and I want to add an ext. speaker. Do I need to go into the amp itself or can it be external?

I see no reason to make it internal. That would be the best way, but you can cut the speaker wire and put in inline. SS amps aren't a finicky when it comes to speaker impedance. That said, I'd still make it a switching jack that cuts off the amp's speaker when you plug in another cab. And I'd make sure the cab you wish to use is the same impedance as the amp's speaker. It isn't that hard to drill a hole in the amp chassis and make it internal. You still would be wiring it the same way. It would just look neater and reduce the potential for damage to almost nil.