Help! Navman 550 Not Picking Up Satellites!

[mischief]

Hi the yacht I buying has a navman 550 with it but it does not pick up satalites? The unit is under the dodger outside with an external antenna that the owner says is the problem. Do these also have an internal built antena if so is it overridden if an external is plugged in?
Finally the cost here in Oz is $280 for the antenna but there is a garmin with BNC connect I can buy for $80 so could I use a different brand of antenna or are they brand and model specific?

Thanks