4 wheel steering issue

I am building a rover and used a 4wheel drive rc car as a base. This car is able to turn both front and rear wheels to have a low radius turn. But for the moment, i am unable to make the rear steering working. Front steering is connected to output 1, throttle to 3 and rear on output 5.
What do I need to do ? throttle and front steering are working (steering is control by Roll, channel 2)

Here is my setup for the inputs (RC is a FlySKY FS i6)
RC channel 3 - Throttle, connected on input 3
RC channel 1 - Roll, connected on input 1
RC channel 2 (normal not used), - Yaw, connected on input 2

What’s the problem ?
Best regards

How is this car supposed to be set up with just an RC receiver. I’m guessing both servos would be plugged into the steering channel of your receiver. No? And what controller are you using.

I am using APM2.6 with a cheap 3 channels receiver. I want to have steering servos mixed by the controller. I could plug both of them on the same output with a reverser but I want the controller to do the mixing.

Replace the obsolete APM with a Pixhawk or FC variant that will run the latest ArduRover. Then set 2 servo output channels to Ground Steering and configure the direction and range you need for each.

If you want to do some interesting mixing (both, front, rear, crab, etc.) like some of the dual steer RTF vehicles have get a better RC system.