Hi
The answers you're looking for are probably in here:
RenderBlocks.renderBlockRedstoneWire()
There is a section near the top that figures out how many neighbouring redstone blocks there are:
boolean flag = BlockRedstoneWire.isPowerProviderOrWire(this.blockAccess, par2 - 1, par3, par4, 1) || !this.blockAccess.isBlockNormalCube(par2 - 1, par3, par4) && BlockRedstoneWire.isPowerProviderOrWire(this.blockAccess, par2 - 1, par3 - 1, par4, -1);
boolean flag1 = BlockRedstoneWire.isPowerProviderOrWire(this.blockAccess, par2 + 1, par3, par4, 3) || !this.blockAccess.isBlockNormalCube(par2 + 1, par3, par4) && BlockRedstoneWire.isPowerProviderOrWire(this.blockAccess, par2 + 1, par3 - 1, par4, -1);
boolean flag2 = BlockRedstoneWire.isPowerProviderOrWire(this.blockAccess, par2, par3, par4 - 1, 2) || !this.blockAccess.isBlockNormalCube(par2, par3, par4 - 1) && BlockRedstoneWire.isPowerProviderOrWire(this.blockAccess, par2, par3 - 1, par4 - 1, -1);
boolean flag3 = BlockRedstoneWire.isPowerProviderOrWire(this.blockAccess, par2, par3, par4 + 1, 0) || !this.blockAccess.isBlockNormalCube(par2, par3, par4 + 1) && BlockRedstoneWire.isPowerProviderOrWire(this.blockAccess, par2, par3 - 1, par4 + 1, -1);
{etc}
par2, par3, par4 correspond to [x,y,z].
Based on these flags, the rendering code chooses the right icons to render (using tessellator.addVertexWithUV()...)
See also http://greyminecraftcoder.blogspot.com.au/2013/08/the-tessellator.html