Bootcamp Windows 7 Installation on Macbook Pro Late 2008 with MiniDisplayPort Monitor (Broken Laptop Display)
I can not guarantee this is going to work. Good luck!


  I assume Windows is detecting your broken display and does not know the display is broken. I assume broken means you can not see anything on the display. I also assume the external display is not mirroring the broken display. If all of this is true, then the problem is: How to get Windows installed to a point where the external monitor can be set as the main display. 


There is an old answer that would require you to install VirtualBox 5.2, which would be compatible with Yosemite. Ignore the update regarding an improved procedure. The following steps would need to be added to the instructions.


At step 23, when running the sysprep application, Choose Enter System Audit Mode instead of Enter System Out-of-Box Experience (OOBE), as shown below.


At step 25, after Windows starts, execute the following steps.


On the external monitor, right click and select Screen resolution, as shown below.


If a window similar to the one shown below appears on the external monitor, then skip to the next step. Otherwise, while holding down the  option key, press the space bar. Release both keys. Next, press and release M key, followed by the → cursor key. Finally, wiggle the mouse in the external monitor display, until the window appears. If the windows does not appear, use the cursor keys to move the window to the external monitor.


In the window, click on the second display, then check off the box Make this my main display, as shown below. Next, click on the OK button to close the window.


Execute sysprep again, but with the following settings.