remote start using relays

https://www.the12volt.com/relays/page5.asp#remst

Some vehicles require that 2 accessory wires go positive to allow Air Conditioner and defrost functions become active when remote started.  If your vehicle only has one Acc wire, leave the second one open.  Yes you will have to cut the ignition wire. 

The unit requires a ground pulse to disengage.  I would HIGHLY recomend that you use an 11th relay that will convert the foot brake positive output to a negative for this function.  This will kill the motor if someone gets in it and tries to put it in gear to drive off. 

On any Store Bought Remote Start unit, there is a safety feature that kills the engine if the brake pedal is pressed before the key is turned on.  This prevents a possible thief from putting the vehicle in gear.   yOu switch on the the engage wire and now the car is running and to turn off the car you switch on the disengage wire. When the unit starts the car, there is power on the ignition wire and both accessory wires. 

You will have to use a relay to convert the positive output of your brake switch to a negative input for the remote start disengage wire.  Wire the relay as follows.  Ground terminals 30 and 85.  86 connects to the wire of your brake switch that has 12 volts on it only when the brake pedal is depressed.  87 connects to the wire labeled negative trigger to deavtivate wire.

I'm with Kevin on this one especially from the tone of your questions and the fact that Mr. I needs his Crystal Ball returned because you haven't told us your make, model and year of car,  there are also all sorts of safety issues going on here, such as type of transmission, transponder by-pass etc. Suggest you have the store bought unit professionally installed. 

Mr I you've copied my diode solder method, as I said looking at Kevins install recently, it's amazing how many similarities in our methods, tools etc.

If you already have an alarm with a AUX output you can buy any brand of remote start that has an activation input - I would think you could get one for much less then 100 pounds.

How are you going to control crank time?  That setup requires a "manual" crank ie you have to hold the output active until the car start, then shut the output off or the starter will continue to crank.  This relay setup has numerous shortcomings.....