Cant write out the exact specific details here on public forum, but would give few pointers.
Security system should be installed in hard to reach place and utilize techniques not commonly used. Like for instance utilizing the neutral start switch in automatic car with a remote operated relay concealed properly with wiring harness piping like oem wiring. Obviously installing a system in such a way is not a small time task.
The way usual shops install security system is just tap their most favourite line at a eaay to reach place and put the system at hands access under the dash.
One if the system i installed requires complete dash removal to bypass/hotwire it. That's how toyota have positioned a crtical component of tbeir immobilzer system in most cars,behind the ac evaporator core,a place one can't reacg without opening the complete dash