You need to be concerned about backfeeding the rest of the electrical system, which is of course what would happen if you just "jumper" in a live feed to the power side of the windows. 
  
The problem with a diode is the amperage being drawn by the windows motors. If you can find a large enough diode cheap, then it can be a good solution. 
  
In the following, the fuse panel and ignition switch may be reversed, 
but my manual is too far away for me to check it. Works the same. 
  
Always disconnect battery before wiring. 
  
Solution (1) 
Add diode, fuse and wire (same or larger size wire as org) as shown. 
Now; 
Battery----Fuse_Panel----Ign_Switch---------------->Windows  
Change to; 
Battery----Fuse_Panel----Ign_Switch-------diode>--@-->Windows  
add the following; 
Battery------Fuse (inline fuse) ------------(splice to @ above). 
Note: Place diode so it only affects the windows power wire. Note polarity. 
  
Solution (2) 
Add switch, fuse and wire (same or larger size wire as org) as shown. 
Now; 
Battery----Fuse_Panel----Ign_Switch---------------->Windows power wire 
Change to; 
Battery----Fuse_Panel----Ign_Switch----(a)switch 
                                                        switch(--->Windows power wire 
Battery------Fuse (inline fuse)-----------©switch 
add the above new fused wire to connector ©. 
  
Note that the wire to the power windows is cut and the end going to the windows is attached to the center ( (or common) connector of  
the switch. The other end of the cut wire coming from the fuse  
panel is connected to switch connector (a). 
  
Toggling the switch determines whether windows ( get power from the 
ignition switch (a) or directly from the battery ©. 
  
Switch is a heavy duty (maybe 12v at 30 amps or more) single throw  
double pole switch. That just means it has 3 connectors, (a), ( and ©.  
  
Solution (3), make windows powered at all times. 
Add fuse and wire (same or larger size wire as org) as shown. 
Now; 
Battery----Fuse_Panel----Ign_Switch---------------->Windows  
Change to; 
Battery----Fuse_Panel----Ign_Switch--------(cut wire)------>Windows  
Tape up the cut end of the old wire from the fuse panel above. 
add the following; 
Battery------Fuse (inline fuse)-----------(splice)--->Windows  
  
SubaPurple