The driver will undoubtedly think he/she is the most important person on board but that's a fallacy that you will have to deal with.
A good way to do this is to include the driver in at least some of the problem solving during the planning stage and keeping him/her involved anytime a decision is needed.
Making it their job to help spot clues, passage controls etc will also help.
Good teamwork is an essential part of a successful road rally or trials crew. Most people stay together as a team wherever possible and build up a rapport, so each knows almost instinctively what the other is thinking. A driver and navigator need to build up trust and confidence in each other's abilities to help them relax and perform at their best.