When you say remove the dash I presume you are talking about the piece right under the handlebars. On the 650 that is easy to remove, just remove the handlebar covers and 8 screws and lift it off. On the 400 it is more complicated.
The video Fernando linked to is for the 07+ so it is not directly compatible with your 04 400 put the parts you will have to take off are pretty much the same. Suzuki calls that part the rear meter panel. To get to it you have to remove the handle bar covers, the front leg shield cover, the windshield, the front leg shield, the front meter panel and finally the rear meter panel.
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