Hmm. I had a similar problem, but my Dell 3137 seemed to have a Dell Wireless-N 1705 card. Windows 7 recognized the BT "half" of the card, but I was on my own to find drivers for the WiFi half of the card. You have a 1704, yet it looks like our PCI Hardware IDs are identical. I got my 1705 WiFi driver from here. Credit to this website. You can search the page for your PCI hw ID if you're still looking.    
Dell Inspiron 11 3137 - DW1704 Wi-Fi + BT Driver = BCM43142 Windows 7 drivers required Does anyone have access to the correct drivers that will work on Windows 7 (32 bit)? This laptop has only been release with Windows 8. All similar Dell products that have the DW1704 wireless card do not contain or have incompatible version of the combo wireless/Bluetooth driver set.
Dell Inspiron 11 3137 - DW1704 Wi-Fi + BT Driver = BCM43142 Windows 7