Installing Alarm and general electrical suggestion please

[Halfcast]

I have a GPX auto and am wanting to install an alarm system and central locking. Now there is currently a hacked up alarm system in there although it's not connected/used but wiring is still there as well as control boxes. Same with the central locking. actuators are there wired up but no remote/not used etc.
I will get the audio/alarm workshop to replace the alarm system and central locking but I have also noticed there is some wiring that would need to get looked at (unsure exact location) as I suspect that there might be an earth that is shorting,causing light flicker some comfort components not operating general old car neglect syndrome.
Is it worth doing the alarm system first as they will pull out all the old crap for me or should I take it to an auto electrician first?
My thought would be do alarm first that way they can just pull out all the old crap and then less diag time needed for the auto elec but that is just me.