I currently have a SWI-PS in my '98 Neon, running to my 790BT. I have many of the same issues. Sometimes my volume/track select buttons work great, sometimes I have a delay. I have mine set up to do Volume up/down, track up/dn (or station up/dn manual), mute and source. My 90BT should be here on the 8th, so I will start the install then.
I really wish I could program the current source button to activate the voice recognition, tho.
Jay