battery backup, unique wiring case?

(Please - newbie here, I've searched through posts but my case may be unique)

I'm adding a 2-way alarm to an old car and would like to power the alarm system exclusively from a backup battery (bub.)   What makes this unique is that I use a master interrupt switch off the main battery ground for security and storage.

Question is, can I isolate the bub/alarm from the chassis ground?  If the bub needs a chassis ground to recharge when the main battery power switch is on, how do I keep the alarm system/bub ground separate from the chassis ground so it doesn't become a bypass of my master switch?  

Could I put the alarm system/bub ground through a relay that permits chassis ground only when the master switch is on?   In this mode, the alarm system could be effectively grounded by the bub only, operating fully independent of the car's electrical system.  Should I put a diode on the power to the bub so power doesn't flow back to the car and deplete the small battery?

And if I can sneak in just one more question - the alarm/bub would be stored near an existing stereo amp (Sony 400w XPLOD) which is fuse-linked direct to battery using 8 gauge wire.  It would be real convenient if I could tap this as a power source to the bub or would the current draw be to much so I should I run another line? 

Any wiring diagrams would help me out tremendously!

Also, should I buy a 520T kit or just get a small 12V battery?

Thanks! If I can reciprocate with any stuff on power boats, classic cars/trucks or ACT! software, please let me know!

- Paul

All done!  Worked great.  Picked up a 12V battery, perfect sized, connected to ignition power thru battery "tender" controller to maintain charge of and isolate the bub. 

Simple AND easy. Coolest thing is the entire system is now self-powered, self-contained and even portable.   I got my pager and optional silent alarm, and I can easily use it in the car, truck or boat.  (I might even make another.)

Nobody had any response or help here but thought I'd share the resolve anyhow.

- P