- Your relay is an "SPST-NO". (Single Pole Single Throw, Normally Open) The bottom contacts on your relaly are like the coil on the left of the diagram and the top contacts on your relay are like the switch on the right of the diagram. When you apply power to the coil, the switch closes.
I've just been using and learning a lot about these solid state relays lately (in some cases, the hard way). There is a lot more to think about than a simple switch.
The bottom terminals (3 & 4) control the switching - when voltage and sufficient current is applied, a switch is closed between top terminals (1 & 2). Live wire for the rumble motor must be switched via the top terminals, 1 & 2. You seem to have just connected everything to the control terminals 1 & 2.
Because you are running motors, even small ones, be aware that inductors (motors have them) store a significant amount of current in their coils, which has nowhere to go once you turn them off. Therefore this current can "slosh" (my word) back and forth over the inductor many times, temporarily (for up to tens of milliseconds) increasing voltage on the wires to scary high levels. This can happen at both the switching/motor side and on the control side (as there is an inductor coil there too, as Scott has pointed out).
Fortunately your SSR unit has snubbers built in to protect the relay. However, they are not there to protect your rumble motors, which may have less tolerance than the SSR.
Therefore it may be a good idea to fit snubbers across the inputs to your rumble motors to protect them from damage.
For about $1 or less each, you can buy a pre-assembled generic snubbers (a varistor, capacitor and resistor) that you can connect across the +'ve and -'ve terminals to your motor. Any high voltage peaks get redirected across the snubber, breaking the high-voltage looping across the inductor.
For a very simple snubber, you can attach a simple fast-switching diode, like a IN4148 or IN914 across the terminals (in reverse direction), which again bleeds off any high voltages generated.
Finally, SSRs have a relatively high residual or leakage current of 2-10 mA or so, which can sometimes create issues.
I'm not Scott and not really an expert, just this stuff is all front of mind for me at the moment. This video by Dave of EEVblog fame explains it better than I can.