Hi all - This is my very first post so I hope I'm making it a good one. We just acquired a 1999 400 SD and it only has the basic electronics it came with. We'd like to put in a GPS or radar and I'm wondering what the proper transducer placement would be - thru-hull or transom mount? Or are there other options? The boat is in saltwater, if that makes a difference. Your advice is appreciated. Many thanks.