designed4living

I just went through this and you can do it, pretty easy if the cars are already installed. Go LSPDFR>data>agency.xml and edit it. You can copy and paste the name of the vehicle(s) you want ai to patrol/respond for backup in for each agency.
 
Here's how mine looks for example for LSPD:
 
<Name>Patrol Unit</Name>
            <Vehicles>
                <Vehicle>police</Vehicle>
                <Vehicle>police2</Vehicle>
                <Vehicle>police3</Vehicle>
                <Vehicle>police4</Vehicle>
                <Vehicle>police5</Vehicle>
                <Vehicle>police6</Vehicle>
                <Vehicle>police7</Vehicle>

LSPDFR didn't throw any errors at the end of the log and it merely stopped - often meaning it never actually crashed. 
 
Would you mind sending your RPH log instead? A text document in your main directory. It is just in a much easier format to read!

LSPDFR has not crashed but rather your game. Go into the RAGE settings and increase your plugin timeout threshold to 60,000