It's hard to provide exact instructions when we don't know exactly how you have the SSR and bed wired. Does the + from the power supply go to the SSR first, or to the bed first?

the difference on the out is 0
Are you seeing 0 across the SSR outputs all the time, or only when the heater/SSR is turned on? The SSR acts like a switch. What you should see is near zero across the outputs when the heater/SSR is turned on, but 12V when the heater/SSR is turned off. If you see 0 all the time, there's an open connection somewhere in your wiring between the power supply, SSR and heat bed or a problem in the heat bed.

The voltage difference on the input is 12v
I assume this is when you've tried to turn the heater on? Does it read near zero when you try to turn the heater off? If so, that confirms that firmware, RUMBA, and the control wiring to the SSR are working.

One easy thing to try would be to temporarily jumper across the SSR output terminals or remove both output connections from the SSR and connect them together. Turn on the printer and see if the bed starts to warm up. If it does, and the SSR control input is known to be working, you pretty much know you have a bad SSR. If the bed still doesn't heat up, there's a problem in either the wiring or the bed itself.