Hi Tulkas,
I'm not sure i fully understand how you have the DHB wired so i'll just go through how it needs to be.
See the data sheet here -
http://motec.com.au/filedownload.php/CDS41213%20DHB%20Dual%20Half%20Bridge.pdf?docid=3244The drawing at the bottom of the data sheet to drive two motors in a single direction is how you need to wire it.
Wire your solinoid in place of motor 1 in the drawing and leave the output pin to motor 2 disconnected.
You must wire both the power supply and ground wires as in the drawing, but you can have switched power if you want.
Your PWM signal from your ECU goes to input 1.
Hope that helps.