Wired remote for Pioneer HU

[fratello]

Hi ! I try to build one wired remote for HU Pioneer , using remote control stalk near the steering wheel, from Renault. I wrote the code ; in simulation work fine. But...I don't know how setting ports "b" and what to do when push_button it's press.
In "original" remote, the command are given by putting resistors "to ground". How can I do this with my PIC ? Thanks in advance !

@ DEVICE pic16F628A, INTRC_OSC, WDT_OFF, PWRT_OFF, BOD_OFF, MCLR_ON, LVP_OFF, CPD_OFF, PROTECT_OFF 

   
   Include "modedefs.bas"   ' Serial Protocol
   Define   OSC 4           ' 4MHz 
   CMCON = 7                ' Disable on-chip comparator, PORTA in digital mode
              
   '// Define port pins as inputs and outputs ...
   TRISA  = %00000111
   TRISB  = %11111100

portb = 1
include "c:\pbp\keypad2.pbp" ' see http://www.picbasic.co.uk/forum/showthread.php?t=3250

main:
gosub keypadscan
gosub check
goto main

check:
select case key 
case 1     ; volume down
   trisb.2=0    ; ?????
   pause 200    ; 
   trisb.2=1    ' ?????