Get back in your Runner and head to the South toward the marker to the Snowbound Crossroads. Drive right up to the ribs sticking out the ground. You need to get out and continue on foot. Kill off the Bullymongs that attack you and press on South. Go underneath the bridge and find the burning car that sits to the South of it:
Nearby it you will find a ECHO Recorder with information on Corporal Reiss. He run for the Marrowfield to lose the Bandits after him. Continue to the South from your present location toward him. Go to the Southern edge of the field. You will find Reiss, but he is being beat on by 3 Psychos.