https://wiki.sa-mp.com/wiki/AttachObjectToVehicle
 

//by Seregamil
native IsValidVehicle(vehicleid);
new vehicleAttached[ MAX_VEHICLES ] = { INVALID_VEHICLE_ID, ... };
new vehicleTimer[ MAX_VEHICLES ] ;
stock AttachVehicleToVehicle( vehicleid, to_vehicleid, Float: offsetX, Float: offsetY, Float: offsetZ, Float: offsetA, time = 100 ) {
    if( !IsValidVehicle( vehicleid ) || !IsValidVehicle( to_vehicleid ) )
        return INVALID_VEHICLE_ID ;
    if( vehicleAttached[ vehicleid ] != INVALID_VEHICLE_ID )
        return INVALID_VEHICLE_ID ;
        
    vehicleAttached[ vehicleid ] = to_vehicleid ;
    vehicleTimer[ vehicleid ] = SetTimerEx( \"updateVehiclesWithAttach\", time, false, \"iiffffi\", vehicleid, to_vehicleid, offsetX, offsetY, offsetZ, offsetA, time );
    return vehicleid ;
}
stock DeAttachVehicle( vehicleid ) {
    vehicleAttached[ vehicleid ] = INVALID_VEHICLE_ID ;
    KillTimer( vehicleTimer[ vehicleid ] );
}
forward updateVehiclesWithAttach( vehicleid, to_vehicleid, Float: offsetX, Float: offsetY, Float: offsetZ, Float: offsetA, time ) ;
public updateVehiclesWithAttach( vehicleid, to_vehicleid, Float: offsetX, Float: offsetY, Float: offsetZ, Float: offsetA, time ) {
    if( !IsValidVehicle( vehicleid ) || !IsValidVehicle( to_vehicleid ) )
        return ;
    new Float: x, Float: y, Float: z, Float: a ;
    
    GetVehiclePos( to_vehicleid, x, y, z );
    GetVehicleZAngle( to_vehicleid, a );
    SetVehiclePos( vehicleid, x + offsetX, y + offsetY, z + offsetZ );
    SetVehicleZAngle( vehicleid, a );
    vehicleTimer[ vehicleid ] = SetTimerEx( \"updateVehiclesWithAttach\", time, false, \"iifffi\", vehicleid, to_vehicleid, offsetX, offsetY, offsetZ, offsetA, time );
}  

AttachVehicleToVehicle( vehicleid, to_vehicleid, Float: offsetX, Float: offsetY, Float: offsetZ, Float: offsetA, time = 100 )
DeAttachVehicle( vehicleid )