If it starts in service position it very well could be the seat belt switch Can you hear anything clicking? Does the dash go blank when you try to crank it? The positive battery cables loved going bad on our LS170s. Always kept extras around because they never made it long. Usually they would cause the display and lights to go off when you tried to crank but not always. We went through a lot of starters too, have you tried arching it to make sure it is good? Otherwise the key switches themselves sometimes went bad as did the interlocks, there are a couple other relays on the back of the cab IIRC for starting also. This is from my knowledge of the LS170s which can be different from your 185.b