I spliced into the Green wire on the ignition switch (live key on) and ran the feed to a relay powering a Centech fuse box directly through the battery. All the load is on the fuse box. Advantages are that the load is through the Centech and not the spliced wire, it only operates with the key on, it's fully fused in case of misshaps and great for attaching other bits of kit. I have HiD fog lamps, a GPS and an Autocom through the system. I would not hook up any accessory through the existing wiring without a relay and seperate fuse as the loom is not designed for the extra sustained current.
As for running only, the electrical system does not work this way. Unless you go for something really radical like hooking into the charging system

, ignition position on is your best bet. Have a search here and you will find a dozen different good ideas - some suggest the horn circuit, others the lighting, others like me went for the igniton. Either way, a relay and seperate fuse are essential and very easy to hook up.